An aromatic polyimide article such as an aromatic polyimide film is known to have high heat resistance and high mechanical strength and therefore is widely employed, for instance, as a substrate of electronic device for incorporation into camera, personal computer, or liquid crystal display. The aromatic polyimide article has, however, such disadvantageous features that a conventional adhesive is not well fixed to its surface and further a spattered or vacuum deposited metal layer is not formed on its surface with enough bonding strength.
Japanese Patent Provisional Publications No. 59-86634 and No. H2-134241 describe that a surface of an aromatic polyimide film can be treated with plasma discharge to increase its adhesion to other material. The plasma discharge treatment is disadvantageous for industrial use because its gives poor productivity. Moreover, the increase of the adhesion property is not satisfactory.
U.S. Pat. No. 5,151,304 describes that the surface of a substrate made of substantially crystalline polyimide is converted to form a layer of substantially amorphous polyimide by treating the crystalline polyimide surface wit a base followed by treatment with an acid to form polyamic acid layer, which is reimidized to form a substantially amorphous layer by a low temperature heat cycle, and metals or polyimides can be deposited onto this amorphous layer, which is then cured to convert the amorphous layer to substantially crystalline polyimide. The United States Patent further describes that the process enhances the adhesion of the metal or polyimide layer. In the working examples, it is disclosed that a film of an aromatic polyimide having a pyromellitic acid unit as aromatic tetracarboxylic acid unit is satisfactorily treated in that manner to give an enhanced adhesion to other aromatic polyimide.
According to studies by the present inventors, the above treatments to form an amorphous polyimide surface are not applicable for forming an amorphous polyimide surface on an aromatic polyimide film having a biphenyltetracarboxylic acid unit as aromatic tetracarboxylic acid unit. Moreover, the curing for converting the amorphous polyimide surface to a crystalline polyimide surface after metals or other polyimides are deposited sometimes lowers the quality of the resultant composite film.
U.S. Pat. No. 5,262,227 describes an aromatic polyimide composite comprising two different kinds of aromatic polyimides arranged in united layers by simultaneous extrusion. One aromatic polyimide layer is derived from an aromatic polyamide acid comprising a tetracarboxylic acid unit and a phenylene diamine unit, while another aromatic polyimide layer is derived from an aromatic polyamide acid comprising a tetracarboxylic acid unit and an aromatic diamine unit having plural benzene rings. The surface of the latter polyimide layer is stated to show good adhesion to a metal film when they are combined under pressure at 280.degree.-550.degree. C.
The copending U.S. patent application Ser. No. 08/690,107, now U.S. Pat. No. 5,741,598, describes a polyimide/metal composite sheet in which the polyimide sheet generally comprises an aromatic polyimide substrate having a thickness of 15 to 150 .mu.m and an aromatic polyimide coat having a thickness of 2 to 10 .mu.m. The aromatic polyimide of the substrate comprises 3,3',4,4'-beiphenyltetracarboxylic acid unit, and the aromatic polyimide of the polyimide coat comprises 2,3,3',4'-biphenyltetracarboxylic acid unit.